The present invention relates to a method and apparatus for warming a gas turbine engine rotor and, in particular, to a method for controlling the gas turbine rotor temperature during periods of extended downtime using steam to heat air extracted from a gas turbine enclosure which is then fed directly to the rotor. In an alternative embodiment, the method utilizes auxiliary boiler steam for purposes of heating the air fed to selected rotor passages during extended periods of downtime.
Gas turbine engines typically include a compressor section, a combustor section and at least one turbine that rotates in order to generate electrical power. The compressor discharge feeds directly into the combustor section where hydrocarbon fuel is injected, mixed and burned. The combustion gases are then channeled into and through one or more stages of the turbine which extracts rotational energy from the combustion gases. The temperature of gas turbine rotor blades rises very quickly when a gas turbine is started because the blades are exposed to very high-temperature exhaust gases. The temperature of the outer peripheral parts of a turbine also increase very quickly due to heat conduction from the blade as compared to inner peripheral rotor components. The rate of increase in temperature thus tends to be slower on the inner side of the rotor than on the outer side. The difference in conductivity of components can also cause a temperature differential between the inner and outer peripheries of rotor components, creating additional thermal stresses during startup. A separate centrifugal stress also exists during startup due to rotation of the engine.
Thus, the combination of thermal and centrifugal stresses on the rotor are much higher when the engine has been sitting idle during, for example, periodic maintenance. As a result, during startup following extended periods of downtime, the rotor disks can undergo significant thermal and mechanical stresses and are vulnerable to premature failure due to the shock occurring during startup, particularly at or near the rotor disks.
An example of a conventional rotor warming structure for a combined cycle plant includes a central gas flow passage with gas from a compressor fed into the central passage in the rotor. Normally, a portion of the compressed gas is introduced into the gas turbine blades through branches emanating from a central passage. Another known method for warming the rotor prior to startup relies on an electrical heating system surrounding the rotor. However, such systems can be prohibitively expensive and often do not sufficiently protect against temperature differentials during startup. Both air and electrical systems also do not take advantage of the potential heating and cost-saving benefits using on-site steam available within the same power generating plant.